The aim of the present proposal is to obtain more information of natural killer or NK cells in mice and men. We would like to develop reagents allowing the selective purification of NK cells or NK susceptible targets from heterogeneous populations and likewise to purify the relevant molecules responsible for the interactions between NK cells and their susceptible targets. We would also like to determine the optimal conditions for via externally administered reagents such as interferon maintain high NK levels in vivo. Finally, we would then analyze the actual in vivo relevance of NK cells as assessed in tumor resistance, marrow grafting or certain infectious diseases. The methods involve lectins of specific binding properties, cellular fractionation procedures taking advantage of surface markers on subgroups of cells, genetical and surgical maneuvers as to create histocompatible animals with distinctly different NK levels for further studies.